Inductive interactions between cells regulate many aspects of vertebrate development, including formation and patterning of the nervous system. Failures of inductive signaling give rise to a wide variety of birth and developmental defects. Despite the obvious importance and many types of inductive interactions required for vertebrate development, the cellular and molecular mechanisms that regulate such interactions are, for the most part, poorly understood. This proposal seeks a more detailed understanding of a developmental signaling process that induces formation of the first muscle cells, termed muscle pioneers. Analysis of zebrafish embryos has shown that muscle pioneers arise and take on their specialized cell-fates, distinct from other muscle cells, as a result of inductive influences from the notochord. To learn when and where these interactions between the notochord and muscle occur, notochord precursors cells, will be a) removed by laser ablation at various times during the inductive process, b) followed with fluorescent dyes in live developing embryos, and c) grown in culture in the presence of muscle precursors. To understand the genetic regulation of the induction process, interactions between two mutations, ntl and spt, that perturb formation of muscle pioneers, will be characterized. The induction of muscle pioneers will be assayed after transplanting a) wild-type notochord precursor cells into ntl mutants, which lack notochords, b) notochord precursors from spt mutants, which lack muscle pioneers, into ntl mutants, and c) wild-type precursors into ntl/spt double mutants. By determining the conditions for rescue of notochord, muscle pioneers, and both, the precise cellular requirements for the induction and the functions of the genes identified by these mutations will be learned. To begin identifying the molecular genetic mechanisms underlying the inductive process, genes expressed in muscle pioneers in response to notochord induction will be characterized by analyzing their promoters and their functions in transgenic fish, and new mutations that affect the formation of muscle pioneers and muscle patterning will be recovered and characterized.